Influences of the electroless nickel electrode on the electrical characteristics of BaTiO3-based PTCR ceramics

This paper investigated the influences of the microstructure of electroless nickel electrode on the a.c. shock characteristics of BaTiO3-based PTCR ce ramics and the degradation and failure of PTC thermistors with electroless nickel electrode. In the experiments, we coated metal nickel electrode on t he: surface of PTCR ceramics samples by employing the different electroless plating baths and analyzed the microstructure of electroless nickel electr odes by SEM. The experimental results indicated that the various baths made the microstructure of nickel electrodes different. The nickel electrodes f rom acid bath possessed the densified stacked structure, which resulted in being cracked when being shocked by 220 V, 3 A a.c. power: owing to having sparse branch-and-tendril structure. those from alkaline bath had no effect s on the ceramics samples being shocked. The work came to the conclusion th at the microstructure of electroless nickel electrodes varied the thermal s tress in the body of PTCR ceramics. In addition, the a.c. shock characteris tics of BaTiO3-based PTCR ceramics were related to the content of phosphoru s in the electroless nickel-phosphorus alloy and thickness of the nickel el ectrodes. According to the experiments, the thickness of the electroless ni ckel alloy should be less than 1 mum and the phosphorus content. 4 wt.% in the alloy. (C) 2001 Elsevier Science Ltd. All rights reserved.